1. Field of the Invention
The present invention relates to a light receiving and emitting unit constituting an optical system of an optical pickup, in which beams emitted from at least two light sources having different wavelengths are collected on an optical recording medium, and the returned beams from the optical recording medium are received, and then recorded on and reproduced from an optical recording medium, more specifically, an optical disk of a different standard.
2. Description of Related Art
In order to downsize a light receiving and emitting unit, a reduction of the number of output lines of the light receiving and emitting unit is one of the effective methods. It is also applied to the light receiving and emitting unit having a plurality of light sources of different wavelengths.
FIG. 10 is a cross sectional view of an optical pickup having a complex optical unit disclosed in Japanese Patent Unexamined Publication No. 2001-101703 (Document 1).
In this document 1, emitted beams from a plurality of light sources (three in the drawing) of different wavelengths are illuminated onto an optical disk D1 (D2), and the returned beams are guided to a predetermined light receiving position on a single light receiving element using two diffraction gratings. Since the angle of diffraction varies depending on the wavelength, the three returned beams each having different wavelengths are diffracted in the three directions at one of the diffraction grating 55f. However, by disposing the other diffraction grating 55d, a difference in the angle of diffraction may be corrected. At the diffraction grating 55d, three beams each having different wavelengths are diffracted again at different angles depending on the wavelength so as to cancel misalignment of the directions of the diffraction grating 55f and guided to a predetermined position of a light receiving element 54.
It is also known in the art that the effects of variations in wavelength are cancelled by applying the above-described technology in which two diffraction gratings are used to correct the difference in diffraction angle.
There is also a known method in which the positions of the light sources are shifted in order to reduce the number of the diffraction gratings to one, and beams from the light sources are obliquely incident onto the diffraction grating at different angles according to their wavelengths (usually they are incident perpendicularly thereto), thereby canceling misalignment of the angles of diffraction for each wavelength.
In order to construct a light receiving and emitting unit to be used for an optical pickup, it is necessary to diverge the optical paths of beams emitted from the light sources and the optical paths of beams returned to the light receiving element. The same is true for the light receiving and emitting unit of multiple wavelengths in which a plurality of light sources of different wavelengths are used.
In the case of the light receiving and emitting unit in the above-described document 1, part of the returned beams from the optical disk is diffracted to the desired directions and guided to the light receiving element by a diffraction grating in order to diverge the optical paths of the beams emitted from the light source and the optical paths of the beams returned to the light receiving element.
In Document 2 (Integrated electro-optic lens/scanner in a LiTaO3, single crystal, APPL. OPT./Vol.38, No.7, p.1186-p.1190[1999]), a device having a light deflecting function and a lens effect by electro-optic effects is disclosed. In this light control device, beams can be collected at a desired position or be deflected at a varying direction without performing mechanical alignment. Since diffraction phenomena are not utilized, deterioration of the performance due to wavelength is prevented.
FIG. 11 shows the light control device presented in the document 2 described above.
In this light control device, there is a difference in a refractive index between the area of lens shape and its surrounding area through the application of a voltage by forming a domain inverted construction (construction in which the direction of deflection is inverted) in triangular shape or lens shape. As shown in FIG. 11, electrodes are formed directly on a crystal thin plate, which is formed of LiTa2O5.
As is described above, in a light receiving unit, it is necessary to diverge the emitted beam from the light source and the returned beam to the light receiving element, and in the technology presented in document 1, diffraction is utilized. However, in the light receiving and emitting unit in document 1, it is difficult to achieve 100% of diffracting efficiency of the diffracted wave at a diffraction order in which the beam can be diffracted to a desired direction. In other words, when the required diffraction order is the first diffraction, minus first diffraction also occurs, and thus unnecessary diffracted beams tend to be stray beams.
Since the incidence points of the optical signals on the light receiving element, which are detected by the light receiving element, may vary due to variations in wavelength, signal detecting operation of the light receiving and emitting unit may be unstable. In addition, since the incidence points of the signal light (detected by the light receiving element) vary due to variations in wavelength, the signal light detected by the light receiving element is affected thereby.
On the other hand, when the light receiving and emitting unit is constructed using the refracting effect, effect of stray light or the effect of variations in wavelength may significantly be eliminated. However, only by diverging the optical paths using the refracting effect, beams after divergence of the optical path cannot be controlled.
In addition, since astigmatism, which occurs when the optical path is diverged, cannot be eliminated from the beam emitted from the light receiving and emitting unit, astigmatism remains as it is.
Furthermore, mechanical adjustment of the incidence points on the light receiving element is necessary when assembling of the light receiving and emitting unit in either configuration using diffraction or refraction for diverging the beams. However, in this case, when attempt is made to adjust the incidence points for beams of a plurality of wavelengths, the diffracting angle may be changed subtly according to the wavelength due to the influence of the manufacturing error of the diffraction grating, or the direction of refraction may be changed subtly due to machining errors of the member for refraction. Therefore, it is difficult to adjust the incidence points to the suitable positions for beams of all wavelengths.
On the other hand, it is conceivable that the light control device shown in the aforementioned document 2 is employed as a light control unit in the light receiving and emitting unit, and that the beam is collected or guided to a desired position for every wavelength. In this case, it may be considered that the necessity of mechanical position adjusting process may be simply eliminated. However, when the above-described device is introduced into the light receiving and emitting unit as it is, the following problems may arise.
In the case of the light receiving and emitting unit, since the inward beam and the outward beam pass the same light control unit, it is necessary to allow the beam to pass through a crystal thin plate, which corresponds to the light control unit, both on the inward and outward routes without loss of light. There is also a requirement to reduce the thickness of the crystal thin plate so as to increase efficiency of light control in the crystal thin plate. In this case, when the crystal thin plate is too thin, mechanical adjustment of the position of the light control unit itself is necessary for allowing the beam to pass therethrough both on the outward and inward routes, and thus mechanical alignment cannot be eliminated when manufacturing the multi-wavelength light receiving and emitting unit. In addition, an increase in thickness of the crystal thin plate only for eliminating the mechanical alignment for both of the outward and inward routes directly leads to the deterioration of performance of the light control unit.
In addition, when attempt is made to bring the device into contact with the same member which only serves to refract the beam for use, the member to be contacted also has an electro-optic effect. Thus, a voltage applied to the device disadvantageously affects to the portion which does not need to be controlled (that is, the same member described above).
In an example in document 1 described above, a light source is located at a position away from the area where a voltage for controlling the optical path is applied (voltage applied portion). However, when the light source is placed in the vicinity of the crystal thin plate, application of a voltage, which is irrelevant to a control of the light source, is performed in the vicinity of the light source. Therefore, when a semi-conductor laser is used as a light source, there arise a necessity of another countermeasure for a problem in that laser destruction may be resulted due to voltage application such as electrostatic induction.